This invention relates to charge coupled devices and more particularly to charge coupled imagers. Basically, a charge coupled device is comprised of a semiconductor substrate having dopant atoms of one type and having a first surface. Charge transfer channels are defined in the substrate near the first surface. An insulating layer lies on the first surface; and a plurality of phase electrodes lie thereon. The phase electrodes are serially disposed over the charge transfer channels. Clocking signals are applied to the phase electrodes for propagating minority carrier charge packets along the channel toward an output end.
In a charge coupled device imager, a plurality of photoelectric charge accumulation sites (picture elements) are also defined in the substrate. Light waves of a predetermined frequency band (e.g., visible light) are directed to fall either on the front side or back side of the device; and minority carrier charges are generated within the substrate in proportion to the intensity of the incident light. These photon generated charges are accumulated in packets at the photoelectric charge accumulation sites. Subsequently, the charge packets are read out via application of clocking signals to the phase electrodes.
In the past, one form of charge coupled device imager had a "frame transfer" type of structure. This structure is described for example in U.S. Pat. No. 3,801,884 entitled, "Charge Transfer Imaging Devices" which was issued to Sequin on Apr. 2, 1974. As therein disclosed, the frame transfer imager is comprised basically of a sensor, a temporary storage array, and a parallel-serial register. The sensor includes an array of photoelectric charge accumulation sites, which may suitably be arranged in a number of rows and columns. The temporary storage device includes the same number of storage sites. In operation, photon generated charge packets are accumulated in the sensor during one time interval; and these charge packets are transferred from the sensor to the temporary storage during a second time interval. This charge transfer occurs in parallel from each column in the sensor. Phase electrodes overlie both the sensor and the temporary storage, and clocking signals are applied thereto in order to accomplish the charge transfer. Subsequently, another set of photon generated charge packets are accumulated in the sensor; while at the same time, the previously accumulated set of charge packets is read from the temporary storage device via the parallel-serial register to an output amplifier.
Another form of prior art charge coupled device imager is of the "interline transfer" type. This is described for example in U.S. Pat. No. 3,826,926 entitled, "Charge Coupled Device Area Imaging Array" which was issued to White et al on July 30, 1974. Basically, the interline transfer imager includes a plurality of photoelectric charge accumulation sites which are arranged in spaced apart columns. Lying within the spaces between the columns are respective parallel-serial charge coupled device registers. Each of these registers have inputs coupled to receive charge packets in parallel from the photoelectric charge accumulation sites, and has one output which couples to a respective input of another parallel-serial register. This other register lies parallel to the output end of the spaced apart parallel-serial registers. In operation, photon generated charge carriers are accumulated at the accumulation sites during one time interval; and subsequently these charge packets are transferred to the plurality of spaced apart parallel-serial registers. After this transfer, another set of charge packets are accumulated; while at the same time, the previously accumulated set of charge packets is read out via all of parallel-serial registers.
One problem with the above-described imager structures is that their physical implementation requires a relatively large amount of substrate space. In particular, the substrate space of the imager is approximately two times the space which is occupied by the photoelectric accumulation sites. This is because the prior art structures use approximately half the substrate space for image sensing, and use the other half of the substrate space for temporary storage of the sensed images. As a result, the total number of charge accumulation sites is substantially reduced over that which could be obtained if all of the substrate area were available for image sensing. And thus the resolution of the sensed picture is reduced accordingly.
Another problem with prior art imagers is that their structure does not enable two charge packets which are sequentially integrated in a particular charge accumulation site to be sequentially read out and compared. That is, in the above-described structures, the charge packets which are integrated during one time interval are all read out; and subsequently, the charge packets which are integrated next are all read out. Therefore, to compare the magnitude of successive charge packets from a particular element requires even greater amounts of storage and/or control circuitry. It is however, particularly useful to be able to perform this sequential readout and compare function. For example, in a moving target indicator, a difference in the magnitude of successively integrated charge packets from a particular picture element indicates movement of the target.
In view of these problems with the prior art, it is therefore one object of the invention to provide an improved charge coupled device imager.
It is another object of the invention to provide a charge coupled device imager wherein almost all of the substrate area can be used for image sensing.
It is another object of the invention to provide a charge coupled device imager wherein charge packets that are successively accumulated in each particular picture element may be sequentially read from the device.
It is still another object of the invention to provide a charge coupled device wherein the charge integration sites are stratified beneath the charge transfer channels.